Electrochemical sensors and biosensors are important for the determination of a wide variety of analytes. However, their performance is often compromised when used in a complex matrix that may contain interferences, or substances that may poison or foul the sensor. Surface-modification of the sensor may reduce or prevent many of these problems. In addition, it can often enhance the operating characteristics of the sensor. One method of surface-modification is to use polymer film/s, composite polymers, or polymer blends to achieve the required sensor characteristics.
An important characteristic of sensor operation is a high degree of selectivity for the desired analyte in a complex matrix. Ideally, the sensor would be a stand-alone unit that is completely specific for the desired analyte regardless of the complexity of the matrix. This would allow a simple measurement of the analyte concentration without any sample preparation or addition of reagents. Unfortunately, this type of operation is rarely, if ever achieved.
Some approaches of how surface-modification of sensors may prevent problems and enhance operation are listed below:
1. Polymer film/s, composite polymers, or polymer blends may form a permselective coating on an sensing electrode that can distinguish between various species by different mechanisms, such as, size exclusion, electrostatic interactions, and hydrophilic or lipophilic interactions. This could eliminate interferences in sensors or biosensors. PA1 2. Polymer film/s, composite polymers, or polymer blends may form a coating on a sensing electrode that can resist or eliminate electrode fouling by unwanted species. PA1 3. Polymer film/s, composite polymers, or polymer blends may form a coating on a reference electrode that can resist or eliminate electrode poisoning or fouling by unwanted species. In addition, the polymer film/s, composite polymers, or polymer blends may used to incorporate species, at a relatively constant concentration, necessary for the maintenance of reference potential, e.g. chloride ion. PA1 4. Polymer film/s, composite polymers, or polymer blends may form a coating on an electrode that is biocompatible. PA1 5. Polymer film/s, composite polymers, or polymer blends may be formed that incorporate materials that assist electrolysis, such as, platinum black particles or other electrocatalytic agent/s to promote the oxidation or reduction of various species, e.g. hydrogen peroxide. PA1 6. Polymer film/s, composite polymers, or polymer blends may be formed that incorporate materials or solvents that locally concentrate beneficial species, such as mediators or oxygen. PA1 7. Polymer film/s, composite polymers, or polymer blends may be used to immobilize different species, such as biochemicals, in close proximity to one another on the same surface. For example, glucose oxidase can be immobilized on one sensor, and lactate oxidose immobilized on another sensor that is adjacent and in close proximity. This type of spatially resolved immobilization is crucial for the successful construction of a multi-sensor array. PA1 8. Polymer film/s, composite polymers, or polymer blends may be used to construct direct sensing ultramicrobiosensors, which do not rely on differential measurement. PA1 9. Polymer film/s, composite polymers, or polymer blends may be formed that can increase the stability or lifetime of immobilized species, such as biochemicals, e.g. enzymes.
Biosensors are devices for sensing such substances as electrolytes, gases, proteins, enzymes, metabolites, antibodies, and antigens. They find wide application in such diverse fields as clinical chemistry testing, bioreactor monitoring and control, fermentation control, and medical research.
A biosensor of the sort of interest here generally includes a base electrode and a biochemically discriminating element disposed about the electrode. Typically the biochemically discriminating element is provided by a membrane which surrounds the electrode. The biosensor is placed in contact with the sample substrate under investigation, and the membrane isolates and transforms the desired analyte into an electrically active species or otherwise generates an electrical signal, which is sensed and monitored by the base electrode.
Other examples of biosensors are provided by so-called enzyme electrodes, in which the membrane is provided by a composite containing one or more immobilized enzymes. The enzymes catalyze a reaction of the sample substrate to consume or to generate a redox-active species in an amount related to the concentration of the target analyte in the sample substrate. The redox-active species may then be detected by measuring current flow at the electrode mediated by the redox-active species. The development of other membranes for selectively discriminating and sensing other analytes is currently an area of active research.
Paralleling the development of new membranes is the development of improved base electrodes. One such development is the use of reticulated vitreous carbon ("RVC") as a base electrode. See, for example, the work of Wieck, Heider, and Yacynych, reported in Anal. Chem. Acta, Vol. 158, pp. 137 et seq. (1984), in which an RVC electrode is incorporated into an immobilized enzyme detector for a flow injection determination of glucose. RVC presents a complex microenvironment to the substances to be detected by the base electrode. Attempts have been made to modify the microenvironment for improved electrode performance. See, for example, the work of Will and Iacovangelo, appearing in the Journal of the Electrochemical Society, Vol. 131, pp. 590 et seq. (1984), which discloses the electrodeposition of zinc metal onto RVC surfaces for improved performance.
Notwithstanding progress in the development of new sensing agents and electrodes, biosensors--old and new--fail to achieve their theoretical peak performance because of interference of species other than the desired analyte which can contribute to the potential difference or to the current sensed by the base electrode, thereby compromising the selectivity or measurement precision of the biosensor, and because of the fouling of the base electrode with repeated use.